Physics
Planets do not twinkle because

They emit light of a constant intensity
Their distance from the earth does not change with time
They are very far away from the earth resulting in decrease in intensity of light
They are nearer to earth and hence we receive a greater amount of light.

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Physics
Plants are dried up in winter due to frost because

Respiration does not take place at low temperature
There is mechanical loss of tissues and drought condition prevails
Photosynthesis does not take place at such low temperature
Evaporation does not take place

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Physics
What is refractive index ?

It is defined as the product of speed of light in medium and in vacuum.
It is defined as the ratio of speed of light in vacuum to the speed of light in the medium.
It is defined as the ratio of speed of light in the medium to the speed of light in vacuum.
None of above

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